#22998: fix(config): add modelByChannel to allowed channels keys

Fixes #22963 ## Summary - `channels.modelByChannel` was added to the Zod schema and TypeScript types in 2026.2.21 but was not added to the runtime validation whitelist in `src/config/validation.ts` - Configs using `channels.modelByChannel` are rejected with `unknown channel id: modelByChannel`, even though the feature works at runtime - Adds `"modelByChannel"` to the `allowedChannels` set alongside `"defaults"` ## Test plan - [ ] Set `channels.modelByChannel` in config and verify `openclaw doctor` no longer reports invalid config - [ ] Verify per-channel model overrides still function correctly 🤖 Generated with [Claude Code](https://claude.com/claude-code) <!-- greptile_comment --> <h3>Greptile Summary</h3> Adds `modelByChannel` to the runtime validation allowlist in `src/config/validation.ts:240`. The `channels.modelByChannel` feature was added to the Zod schema and TypeScript types in commit f555835b (2026-02-20) but the runtime validation whitelist was not updated, causing configs with this valid key to be rejected with "unknown channel id: modelByChannel" errors. - Aligns runtime validation with the schema definition in `src/config/zod-schema.providers.ts:34` - Matches the existing pattern where `defaults` is also a special channel key - Consistent with usage in `src/commands/doctor-state-integrity.ts:178` which already treats `modelByChannel` as a special case <h3>Confidence Score: 5/5</h3> - This PR is safe to merge with no risk - Single-line fix that adds a missing validation whitelist entry for an already-implemented feature. The change is necessary, correct, and matches the existing pattern for special channel keys like `defaults`. No logical issues, security concerns, or edge cases identified. - No files require special attention _{Last reviewed commit: 945131e} <!-- greptile_other_comments_section --> _{(2/5) Greptile learns from your feedback when you react with thumbs up/down!} <!-- /greptile_comment -->